Silicon integrated devices, because of their small size and fragile nature are frequently embedded in a plastic molding compound and electrically connected to an array of electrical conductors which project beyond the edge of the assemblage. The ends of the electrical conductors embedded within the plastic molding compound and located close to the silicon device are known as the inner leads and are connected to bonding pads on the silicon device through fine gold wire bonds. The outer ends of the leads which project outward from the edge of the assemblage are shaped to form "J" type leads or "Gull Wing" leads for mounting onto a circuit board.
The plurality of leads which connect the silicon device to the circuit board is stamped or punched from sheet material and is called a leadframe. In those instances where the density of the leads is high, the leads may be formed by etching because the spacing between the inner leads may be too small to be formed with a punch tool.
After the leadframe is formed, the inner end of each lead is connected to a bond pad on a silica device by means of a gold or aluminium wire. In some instances, the silicon devices are supported by a ceramic member which, in turn, is physically supported by the inner leads. Thus, the ends of the inner leads are electrically connected to the silicon device and also provides support for the silicon device.
One method of forming the final package is by a "post-molding" process. In the process the leadframe is entirely encapsulated by a thermoset plastic molding compound.
Encapsulation normally occurs in a production-size molding tool over a three minute cycle. Of the three minute cycle time, the plastic molding compound flows into the mold in 15 seconds, two minutes is needed to polymerize or cure the plastic; and the remaining time is used to prepare the mold for the next molding cycle. The plastic molding compound generates flow-induced stresses on the fragile silicon device, the connecting wires and the leads as it fills the cavity which causes damage and yield loss.
Another method of packaging electronic devices is that of a pre-molded package. In this method a plastic ring or shell is molded around a leadframe, the silicon devices are attached to the leadframe in some fashion, and a lid or cover is attached to seal the package. With this method the molding compound does not flow over the silicon devices and wirebonds.
In both packaging schemes, it is critical to maintain coplanarity of the leads after the package is fabricated. In the case of the post-molded package, the thermoset material provides enough mass and the thermal shrinkage is uniform enough so as to assure coplanarity of the leads in the final configuration. In the case of pre-molded packages, a frame member is molded around the leads initially, the silicon devices or substrate is then attached followed by attachment of a cover. The frame is a thin member which must be strong enough to withstand subsequent trim and form operations. In this case the plastic is generally a thermoplastic material and due to this type of molding, it can often contain large stresses which can lead to excessive warping. This warpage can often cause unacceptable warpage of the leadframe thus causing difficulty in attaching the leadframe to a printed circuit board.
This invention is directed towards providing a substantially flat pre-molded leadframe which minimizes the stresses applied to solder joints and resists cracking and/or breaking during assemblage.